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In their paper [Opt. Mater. 24, 333, 2003], Feldman et al conducted anexperimental study of the dynamics of chromium ion valence transformations inCr,Ca:YAG crystals among the trivalent Cr3+ state, and two tetravalent Cr4+ones, of octahedral and tetrahedral coordination. The temperatures used rangedbetween ~800 and 1,000 C. The basic effects are the transition of Cr3+ intoCr4+ under high-temperature annealing in an oxidizing atmosphere, and thereverse transition under a reducing atmosphere, or in vacuum. In the presenttheory, we interpret the processes by oxygen-vacancy diffusion in the bulk ofthe YAG. The quasi-chemical reaction VO2- + Cr4+ = Cr3+/VO- between thechromium ions and the vacancies VO2- is responsible for the valencetransformations. Dynamical analysis provides profiles of the Cr3+ and Cr4+concentrations in the crystal during oxidation and during reduction. Reactionrate profiles are also calculated, establishing the reaction front position andwidth. A comparison with existing experimental results on the integrated Cr4+concentration as a function of time during oxidation [Opt. Mater. 24, 333,2003] shows reasonable agreement.

Theory of oxidation/reduction-induced chromium ion valence transformations in Cr,Ca:YAG crystals